U.S. Pat. No. 1,590,497 and U.S. Pat. No. 1,947,901, by Juan de la Cierva y Codorníu, amongst others, define and protect the autogyro, which is a machine equipped with rotary wings that obtains its main lift in flight from the reaction of air on a system of aerofoils or rotors capable of rotating freely. Thus, one could say that the autogyro is an aeroplane equipped with propeller-shaped wings, articulated on a vertical axis, which rotate as a consequence of air resistance during the machine's forward movement and act as lifting elements.
From the time that the autogyro was invented by Juan de la Cierva y Codorníu in Madrid in 1923 up to the present, all designers of rotary wing machines, primarily autogyros and helicopters, have attempted to expand these machines' range of speeds in order to make them comparable to those of fixed wing aeroplanes. Starting with the first autogyro models, particularly those designed in the United States, they were hybrid designs wherein standard wings intended for high-speed flight co-existed with the rotor, the basic element for providing lift at low speeds.
The efforts to achieve high flight speeds in rotary wing machines have been hindered by the basic fact that a rotor in flight, at relatively high speeds, exhibits a very asymmetric profile in the lift generated by the rotor blade when it “moves forward” in the wind produced by the aircraft's forward flight and when it “moves backward” in that same wind, the opposite side of the rotor disc.
This asymmetric flight profile is very visible if the velocity (with respect to the wind) of the external tip of the rotor blade is analysed. It is easy to see that, when the blade is in a position of maximum forward movement, the velocity is the sum of the aircraft's rotational and translational velocities. On the contrary, when the blade is on the opposite side, the velocity thereof is the difference between both velocities.
Therefore, when a rotary wing aircraft attempts to fly at high speeds, it is possible that the tip of the blade exceeds the speed of sound on the blade that moves forward and/or suffers losses on the blade which moves backward, which leads to highly undesirable effects in the rotor's behaviour.
This factor has limited the maximum speed of rotary wing aircrafts (autogyros and helicopters) to slightly over 350 km/h. This is in contrast with the speed of over 1,000 km/h commonly achieved by fixed wing aircrafts, including civil air transport. This speed is slightly below the speed of sound in air, which at sea level is about 330 m/s, equivalent to about 1,200 km/h.
Numerous military aeroplanes and some civil ones (such as the “Concorde”) reach supersonic speeds, but at the expense of substantial increases in consumption, noise, heating of the fuselage and several other characteristics.
In rotary wing aircrafts, the asymmetry in the lift of a rotor in flight also generates an asymmetric effect due to the “loss of speed” in internal sections of the tips of the blades. Thus, the linear velocity produced by the rotation decreases with the radius, whereas the translational velocity remains constant. For this reason, the area of each blade wherein the velocity falls below the stall velocity is greater when the aircraft's translational velocity increases. The entry into “loss” (“stall” in the field) of an increasingly larger part of the blade that moves backward in the forward wind also produces an asymmetry in the lift of the rotor.
The limited maximum speed of rotary wing aircrafts presents serious restrictions for the use thereof. It is evident that the main incentive—the raison d'être—of these aircrafts is their capacity for slow, stationary flight, as well as their capacity for taking off and landing in a space limited to a size that is slightly larger than the aircraft and the rotor thereof. But many of the civil or military missions wherein helicopters are involved consist of transporting persons and/or cargo between two points, one or both of which may not be equipped with take-off or landing infrastructures. In these cases, the low maximum and cruise velocities of autogyros and helicopters lead to long transport times, which greatly limits their practical use for many missions.
During the eighty years that have elapsed since the birth of rotary wing aviation, there have been numerous attempts to break the high-velocity barrier in these machines. Without exception, they have all been based on hybrid designs composed of wings and a rotor, with the intention to transfer the lift from the rotor at low speeds to the wings at higher speeds. These aircrafts are known as convertible or hybrid aircrafts, or “convertiplanes”.
Thus, currently a large number of convertible aircraft embodiments are known, composed, in a well-known manner, by a fuselage, standard fixed wings equipped with ailerons, a tail unit with rudders, engines, a rotor with blades, a transmission between the engines and the rotor, equipped with braking and rotor clutch means, and a landing gear.
Below we list and describe a substantial part of these embodiments, which, as a whole, define the closest state of the art.
U.S. Pat. No. 1,792,014, by G. P. Herrick, describes an aircraft of this type with lifting wings in a normally fixed position with an assembly that allows for the rotation thereof according to essentially horizontal planes in the form of a lifting propeller driven by moving air and with pivoting movements with respect to the rotational axis. This aircraft also has retention means to retain the wing in a fixed position without the possibility to rotate, retention means to maintain the wing in position with respect to the pivoting, and release means for both retention means at the pilot's discretion, as well as means to drive the aircraft through the air.
Therefore, this aircraft of U.S. Pat. No. 1,792,014, the practical commercial version whereof was called “Herrick HV2A” convertiplane, was an aircraft that could fly as an autogyro and as an aeroplane with the rotor stopped in a transverse position, making several in-flight transitions between the two modes. The HV2A is, evidently, an attempt to overcome autogyros' speed limitations. Its maximum velocity was 160 km/h.
The “Fairey Gyrodyne” is a convertible aircraft, designed by Fairey Aviation Ltd. in Great Britain in 1946. This convertible aircraft is a hybrid aircraft between a helicopter and an autogyro that uses a propeller on the port side which serves to compensate for the torque generated upon applying power to the rotor. In autogyro mode for rapid flight, the same propeller serves to provide thrust to the aircraft. The aircraft reached a maximum velocity of 200 km/h, which at the time, 28 Jun. 1948, set a world record for rotary wing aircrafts.
The original Gyrodyne was extensively modified to be converted into the Jet Gyrodyne (1953) in order to study the principle of jet propulsion of the blades conceived for the Rotodyne, which is described further below. Although the modified Jet Gyrodyne maintained the general configuration of the Gyrodyne, it mounted a two-blade rotor with augmentors on the tips to replace the previously used three-blade type and was equipped with two propellers. Two compressors of the type used in the Rolls-Royce Merlin engine supplied compressed air to the tips of the rotor, which rotated freely, and a Leonides engine was used solely to move the two Fairey variable-pitch driving propellers mounted on the tips of the wings. No data have been found regarding the maximum velocity reached by this design.
Given the positive result of the Jet Gyrodine, the proposal by Doctor J. A. J. Bennet, one of Juan de la Cierva's main collaborators, and Captain A. G. Forsyth, formulated in 1947, to build a large convertiplane, seemed to be promising. In December 1951, British European Airways requested a 30-40-seat machine for short and medium routes, and Fairey submitted a proposal which more or less corresponded to its ideas. It was accepted and, in 1953, the English Ministry of Supplies granted it a contract to build an experimental prototype. The system of trials consisted of a main rotor, the two turbines, wings, etc., and the controls were installed in a compartment located in the approximate position of the nose. Exhaustive tests were conducted whilst the prototype was being built. The Rotodyne made its first flight as a helicopter on 6 Nov. 1957, and the first transition to horizontal flight took place in mid-April of the following year. The Rotodyne had an orthodox quadrangular-section fuselage with short, rectangular wings whereon the Eland turbines were mounted. The tricycle landing gear was retracted inside the engine nacelles. A double fin, subsequently completed with another, central one, was mounted on the tips of the tail aerofoil, which had a rectangular plan and was installed in a high position. Vertical take-off was achieved thanks to a large “four-blade” rotor, with jet propellers on the tips, which were fed with compressed air purged from the turbines and mixed with fuel. They were possibly ram-jet engines. Each turbine fed two opposite blades in order to avoid asymmetries in the event of failure of an engine.
On 5 Jan. 1959, the Rotodyne beat the world record for rotary wing aircraft velocity for convertiplanes in a 100-km closed circuit, setting it at 307.2 km/h.
U.S. Pat. No. 2,702,168, applied for in 1950, discloses a convertible aircraft which may fly in helicopter mode and in aeroplane mode, equipped with wings that extend on both sides of the fuselage, with rotors mounted on the wings capable of oscillating around a horizontal axis, with the possibility to modify the angle of attack and the thrust vectors of said rotors differentially with respect to one another. The practical embodiment of the aircraft disclosed in this patent is Bell-Boeing's V-22 Osprey, which resolves the problems of lift asymmetry in the rotor (or rotors) at high flight speeds, making the same rotors transform in flight in such a way that they act as high-velocity tractor propellers.
This convertible aircraft has a cruise velocity of 432 km/h and the different United States army forces have ordered several hundred units. The maximum velocity achieved by the V-22 Osprey is 510 km/h.
U.S. Pat. No. 5,727,754, by Carter Copter discloses a convertible or hybrid aircraft between an autogyro and an aeroplane, equipped with an autogyro rotor, a variable-pitch driving propeller for propulsion, and wings with a relatively small surface area. The CarterCopter is a convertiplane that is in the process of development in the United States on the date of filing of this patent.
The CarterCopter company has announced its intention to reach high maximum velocities using a technology, called “μ−1”, where μ is the ratio between the forward velocity of the tip of the rotor blade and the linear velocity of the CarterCopter. Carter maintains that, for μ values greater than 1, corresponding to high aircraft velocities, the lift comes solely from the CarterCopter wings and the drag of the rotor, self-rotating at a very low rotational velocity, is also very low, which will allow the aircraft to reach high forward velocities whilst the rotor continues to self-rotate at a low rotational speed and remains stable, assisted by masses installed inside the blades close to the tip.
At the date of this patent, the μ−1 theory has not been verified in flight. The prototype has as yet not reached sufficient velocity to be tested.
As will be shown, all the attempts described, based on hybrid designs composed of wings and a rotor, with the intention of transferring the lift from the rotor at low speeds to the wings at higher speeds, are limited to autogyro aeroplane, helicopter-aeroplane and autogyro helicopter dual hybrids or combinations.
It seems evident that an aircraft which may operate at low or zero velocity as a helicopter, but may reach maximum speeds that are much higher than those of current helicopters, as well as the in-flight safety characteristic of the autogyro, would find a substantial niche in both civil and military markets, filling the void in the current state of the art.
The purpose of this invention is to provide a new convertible aircraft embodiment, as well as an operating method for this aircraft, which resolves the problem posed and fills the above-mentioned void.